The present invention relates generally to optical systems, and more particularly to an improved optical system employing at least one fluid which is frozen onto a transmissive substrate positioned in the optical path of a detection system so as to function as an optical filter, thereby absorbing undesirable atmospheric optical radiation emitted from various sources.
It is generally known that all objects emit infrared radiation. The temperature of an object determines how much radiation is emitted and at what particular wavelength. The higher a body""s temperature, the more radiation emitted and the shorter the peak wavelength of the emissions. As an object""s temperature increases, the location of the xe2x80x9cpeakxe2x80x9d wavelength moves toward shorter wavelengths. For example, the surface of the sun, at 60000xc2x0 K, has its peak in the yellow region of the visible portion of the spectrum, and therefore, appears yellow in the sky. Conversely, a fighter aircraft exhaust, at approximately 800xc2x0 K, isn""t hot enough to emit radiation in the visible spectrum. The fighter aircraft exhaust""s peak emission occurs at roughly three micrometers (mm) and is located in the infrared region of the spectrum.
Similar to the colors of the rainbow, the infrared spectrum is divided into subregions primarily based on how they are utilized in sensor systems. The boundaries of these regions are not absolute, but normal convention breaks down the infrared region into four basic categories: Short, Medium, Long and Very Long wavelength. Just beyond the color red in the visible spectrum, i.e., with a wavelength slightly longer than red, is an area known as Short Wavelength Infrared (SWIR). This band generally covers the wavelengths between 1-3 mm and is used by space based sensors to see the bright rocket plumes of boosting missiles. Slightly longer in wavelength and covering from 3-8 mm is the area known as Medium Wavelength Infrared (MWIR). Space systems use this band to detect and track objects through booster burn out against an Earth background [i.e., Below the Horizon (BTH)]. From 8-14 mm, is an area known as Long Wavelength Infrared (LWIR). The long wave band is used by space sensors to see objects Above the Horizon (ATH) against a cold space background. The final region of the infrared, Very Long Wavelength Infrared (VLWIR), is located beyond 14 mm and typically ends around 30 mm. This band is used to track extremely cold targets against a space background.
Because all heated objects emit infrared radiation, the infrared is an excellent spectral region to use for object detection and tracking. Using an infrared detector, an object""s emitted radiation can be detected, measured and plotted. Since every object has a unique infrared signature or xe2x80x9cfingerprint,xe2x80x9d a positive object identification can be made based on the received energy.
In order to detect the infrared radiation emitted from heated objects, a material sensitive to infrared radiation is needed. Current space based systems use photon detectors in order to xe2x80x9cseexe2x80x9d this thermal radiation. Photon detectors consist of a semiconducting material that is sensitive to infrared radiation. The radiation consists of energy packets called xe2x80x9cphotonsxe2x80x9d that interact directly with the material and generate electrical signals. The detector material is divided into small sections called xe2x80x9cpixels,xe2x80x9d and a detector""s resolution is determined by the size, spacing and number of these pixels. The name given to a material segregated into pixels is a xe2x80x9cSensor Chip Assembly.xe2x80x9d
Today, most SWIR, MWIR, and IWIR detectors are made of either Mercury-Cadmium-Telluride (HgCdTe) or Indium-Antimonide (InSb); however, Silicon (Si) and Germanium (Ge) are still used for VLWIR detectors.
These infrared sensitive materials can be integrated into a larger device called an xe2x80x9cinfrared sensor system.xe2x80x9d An infrared sensor system is a collection of optical elements and electronic hardware connected to an infrared detector. The optical elements reflect and focus incident radiation from an object onto a focal plane, and electronic hardware attached to the focal plane is used to xe2x80x9cread outxe2x80x9d the electrical signals generated by each pixel of the focal plane. Signal processors are used to convert these analog voltage signals into digital images that can be used by a computer to determine which infrared signature(s) the detector is receiving.
On a space based sensor, each detector collects photons from a particular area on the Earth known as a xe2x80x9cfootprint.xe2x80x9d The size of this footprint is determined by the angular field of view of each pixel and the altitude of the sensor. A detector at a high altitude will see a larger area than one at a low altitude; however, a low flying sensor will generally have better resolution.
There are two basic types of sensorsxe2x80x94xe2x80x9cstaringxe2x80x9d and xe2x80x9cscanning.xe2x80x9d In a staring sensor, a square or rectangular Focal Plane Array (FPA) continuously looks at a particular area and watches for changes in the incoming infrared radiation over time. The benefit of this technique is that an area is under constant watch, and depending on how often the electronics read out the incident photon energy on the FPA, it is possible to detect small, quick changes in incident radiation intensities. The drawback is that this kind of focal plane generally needs to be large in order to cover a significant area, and these large arrays are more expensive and difficult to build than smaller arrays.
A second technique is to use a smaller array and scan across a region to build a picture of the entire scene. Some common scanning detector methods include the side-to-side toggle scanner, the line scanner or xe2x80x9cpushbroomxe2x80x9d and the spin scanner or xe2x80x9cspinner.xe2x80x9d The advantage of the scanning sensor is that the FPAs can be manufactured relatively inexpensively compared to large staring sensors while still providing the necessary coverage. The drawback is that as the FPA performs its scanning, it cannot watch an entire scene simultaneously and might miss a change in an event occurring outside its immediate scan area. The speed at which a scanning sensor returns to a particular spot in the field of view is called xe2x80x9crevisit rate.xe2x80x9d If the revisit rate can be made fast enough, a scanning sensor provides a practical alternative to a staring sensor.
The ultimate decision for which type of sensor to use depends on many factors including satellite configuration, mission, altitude and performance requirements.
Infrared sensors are xe2x80x9cpassivexe2x80x9d devices, which means they do not send out and receive signals as do xe2x80x9cactivexe2x80x9d sensors, such as laser or radar sensors. Instead, they passively wait until infrared energy from an object strikes the detector and is measured.
A space based infrared system allows each sensor to view a large area due to its high altitude; however, because satellites are so far away, the infrared radiation needs to travel a great distance in order to reach it, which reduces the amount of radiation received at the detector. In addition, the atmosphere absorbs some infrared radiation at particular wavelengths, thus reducing the amount of radiation reaching the detector even more. To overcome these factors, space based infrared detectors are designed to be very sensitive.
One of the problems in detecting objects through the Earth""s atmosphere (or any intervening medium) is the infrared self-emission of the medium itself. This problem is especially significant for both ground- and space-based sensors looking through the atmosphere.
Earth""s atmosphere contains significant amounts of water, as well as carbon dioxide. The water and carbon dioxide emit energy (e.g., infrared) in the wavelength band which the detector xe2x80x9csees,xe2x80x9d causing a large amount of background light (typically referred to as clutter or noise), with a corresponding reduction in image contrast or visibility. This is equivalent to looking through a dense fog and trying to locate a very faint and distant object moving at high speeds.
The standard approach to filtering out unwanted infrared radiation from the Earth""s atmosphere is to employ an interference filter placed in the optical path of the sensor system. An interference filter is generally defined as an optical filter that reflects one or more spectral bands or lines and transmits others, while maintaining a nearly zero coefficient of absorption for all wavelengths of interest. An interference filter may be high-pass, low-pass, bandpass, or band-rejection. The interference filter typically consists of multiple layers of dielectric material having different refractive indices. There may also be metallic layers. Interference filters are wavelength-selective by virtue of the interference effects that take place between the incident and reflected waves at the thin-film boundaries.
However, the tighter the requirements on the interference filter, the more layers of dielectrics it needs, with the design becoming much more complicated, harder to manufacture, and potentially more mechanically unstable. Furthermore, these interference filters""properties change with temperature, pressure, and angle. Additionally, complicated interference filters can be very expensive to manufacture and maintain.
Therefore, there exists a need for a system for filtering out all, or substantially all, atmospheric infrared radiation so as to prevent, or at least minimize, detection of same by an infrared sensor system, including ground-and space-based systems, thus increasing the resolution and effectiveness of the infrared sensor system.
The following U.S. Patents contain information relating generally to the background of the present invention, the entire disclosures of all of which are incorporated herein by reference:
U.S. Pat. No. 3,906,231 issued to Fletcher et al., discloses a superconductive tunneling device having a modified tunnel barrier capable of supporting Josephson tunneling current.
U.S. Pat. No. 3,982,404 issued to Overbye, discloses an I.Q.F. system for deep freezing of food articles and the like.
U.S. Pat. No. 4,100,760 issued to Cheney, discloses loose particulate material to be frozen is supplied to a rising current of refrigerating fluid such as cold air.
U.S. Pat. No. 4,127,163 issued to Reti, discloses a method and apparatus for freezing and subliming uranium hexafluoride (UF6) as part of a gaseous diffusion plant from which a quantity of the UF6 inventory is intermittently withdrawn and frozen to solidify it.
U.S. Pat. No. 4,324,285 issued to Henderson, discloses an apparatus having a high temperature probe and a low temperature probe.
U.S. Pat. No. 4,396,636 issued to Mitsuda et al., discloses a method for producing a frozen-food.
U.S. Pat. No. 4,448,524 issued to Brus et al., discloses an efficient transmission of light through a matrix isolation thin film is obtained despite the fact that the index configuration of the thin film is not appropriate for classical waveguides.
U.S. Pat. No. 4,478,861 issued to Montgomery et al., discloses a mixture of food pieces that are first cooked.
U.S. Pat. No. 4,748,817 issued to Oura et al., discloses a method for the production of microfine frozen particles.
U.S. Pat. No. 4,829,784 issued to Berg et al., discloses a method and system for storing gas, especially an inert gas for electric impulse space drives which use inert gas as a reaction mass.
U.S. Pat. No. 5,045,703 issued to Wieboldt et al., discloses a gas sample collection device and method for cold trapping individual gas bands from a gas source that may include a chromatographic separation and for spectrographically analyzing the individual gas bands.
U.S. Pat. No. 5,219,005 issued to Stoffel, discloses a twin-chamber container which includes an outer plastic container with a valve mounted thereon and an inner collapsible container mounted in communication with the valve.
U.S. Pat. No. 5,220,796 issued to Kearns, discloses a volatile component is recovered from an inert gas blanketed gas source containing the volatile component, an inert carrier gas, water vapor and oxygen as an impurity by a continuous process.
U.S. Pat. No. 5,328,517 issued to Cates et al., discloses a method for removing material from a structure having at least one layer of material formed on a substrate.
U.S. Pat. No. 5,459,771 issued to Richardson et al., discloses a high repetition-rate laser plasma target source system and lithography system.
U.S. Pat. No. 5,486,373 issued to Holt et al., discloses frozen low bulk density dessert or confection products, such as ice confections.
U.S. Pat. No. 5,577,091 issued to Richardson et al., discloses a high repetition-rate laser plasma target source system wherein ice crystals are irradiated by a laser and lithography system.
U.S. Pat. Nos. 5,514,936, 5,628,831 and 5,696,429 issued to Williamson et al., disclose contaminants are cleaned from the surface of a body in space by generating a substantially space-charge neutral reactive plasma, directing the plasma onto the contaminated surface at an energy below the surface sputtering energy, and reacting the plasma with the contaminants to remove them.
U.S. Pat. No. 5,763,930 issued to Partlo, discloses a high energy photon source.
U.S. Pat. No. 5,782,253 issued to Cates et al., discloses a system for removing material from a structure having at least one layer of the material formed on a substrate.
U.S. Pat. Nos. 5,809,801 and 5,860,295 issued to Cates et al., disclose a method and apparatus for accumulation of hyperpolarized 129Xe.
U.S. Pat. No. 5,869,626 issued to Yamamoto et al., discloses a novel metal-encapsulated fullerene compound wherein a side chain is introduced in a metal-encapsulated fullerene.
U.S. Pat. No. 5,964,043 issued to Oughton et al., discloses a process and apparatus for freeze drying of liquid material in a vessel in which the vessels are moved automatically through various stages.
It is therefore a principal object of this invention to provide a new and improved optical filter.
It is another object of this invention to provide a new and improved infrared optical filter.
It is another object of this invention to provide a new and improved space-based infrared optical filter.
It is another object of this invention to provide a new and improved optical filter.
It is another object of this invention to provide a new and improved infrared optical filter system.
It is another object of this invention to provide a new and improved space-based infrared optical filter system.
It is another object of this invention to provide a new and improved sensor system.
It is another object of this invention to provide a new and improved infrared sensor system.
It is another object of this invention to provide a new and improved space-based infrared sensor system.
In accordance with one embodiment of the present invention, an optical filter is provided, comprising:
a substrate transmissive to infrared radiation, wherein the infrared radiation includes at least two different wavelength bands; and
a layer of material formed on at least a portion of a surface of the transmissive substrate, wherein the material substantially absorbs at least one of the at least two different wavelength bands of the infrared radiation so as to substantially prevent the transmission of one of the at least two different wavelength bands of the infrared radiation through the transmissive substrate.
In accordance with another embodiment of the present invention, an optical filter system is provided, comprising:
an optical system for focusing infrared radiation, wherein the infrared radiation includes at least two different wavelength bands;
a substrate transmissive to the infrared radiation; and
a layer of material formed on at least a portion of a surface of the transmissive substrate, wherein the material substantially absorbs at least one of the at least two different wavelength bands of the infrared radiation so as to substantially prevent the transmission of one of the at least two different wavelength bands of the infrared radiation through the transmissive substrate.
In accordance with still another embodiment of the present invention, an infrared detection system is provided, comprising:
an optical system for focusing infrared radiation, wherein the infrared radiation includes at least two different wavelength bands;
a substrate transmissive to the infrared radiation;
a layer of material formed on at least a portion of a surface of the transmissive substrate, wherein the material substantially absorbs at least one of the at least two different wavelength bands of the infrared radiation so as to substantially prevent the transmission of one of the at least two different wavelength bands of the infrared radiation through the transmissive substrate; and
an infrared radiation sensor system in communication with the transmissive substrate.
These and other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.